Air staged low-NOx burner

ABSTRACT

An apparatus and method for using staged air combustion. The apparatus includes a burner body ( 10 ) secured to a port block ( 42 ), and a fuel passageway ( 12 ) extending through the burner body ( 10 ), terminating in a fuel nozzle ( 22 ), which injects fuel into the burner throat ( 40 ). Primary air jets ( 20 ) are configured to inject primary air into a primary combustion region ( 24 ), which is normally in the burner throat ( 40 ). A dish with a dish surface ( 28 ) is connected to the burner throat ( 40 ); the dish surface ( 28 ) extending in a divergent angle with respect to a burner centerline ( 35 ). Secondary air jets ( 34 ) are connected to the air passageway ( 14 ) and extend through the port block ( 42 ). The secondary air jets ( 34 ) inject secondary air into a secondary combustion region ( 38 ), which may be at the dish surface ( 28 ) or the hot face ( 30 ) of the burner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to low-NOx burners, and, in particular, toair-staged low-NOx burners.

2. Description of the Prior Art

Oxides of nitrogen (NOx) are produced from the burning of fuels duringthe normal operation of a typical burner. These oxides combine withhydrocarbons in the atmosphere, creating “smog”, which, when inhaled,may cause injury. Further, the U.S. Environmental Protection Agency, aswell as state and local air pollution agencies, have passed certainenvironmental laws providing limitations and technological standards onthe amount of NOx a facility may emit. These standards are continuing tobecome more and more stringent, creating a technological need forlow-NOx burners.

Decreasing the NOx emissions from a burner is a well-known need. Forexample, U.S. Pat. No. 4,004,875 to Zink et al. (hereinafter “the Zinkpatent”) discloses a low-NOx burner concept that introduces secondaryair to the hot face of the burner in addition to the primary air. In theZink patent, primary air is provided in an amount that is insufficientto completely combust the fuel. The secondary air is introduced in asecond stage to complete the combustion process. Overall, the use ofstaged air in this manner leads to reduced NOx emissions from the burnerunit. Likewise, U.S. Pat. No. 4,347,052 to Reed et al. discloses the useof primary, secondary and tertiary air in predetermined stoichiometricproportions in order to stage combustion and, thus, reduce theproduction of NOx from the burner. Finally, U.S. Pat. No. 4,983,118 toHovis et al. describes the use of air staging to reduce the productionof NOx from a regenerative burner. The introduction of secondary ortertiary air in all of these burner concepts demonstrates the well-knownusage of incomplete combustion to retard the production of NOx from theburner. This retardation occurs due to the overabundance of carbondioxide, water vapor and methane in the burner mix at the initial stage.

As the environmental laws tighten, there is still considerable room inthe art for technology that further reduces the production of NOx fromindustrial burners. While the above-referenced patents, among others,use incomplete combustion to reduce NOx, improvements over this designconcept are in need.

SUMMARY OF THE INVENTION

The present invention uses staged air combustion to reduce theproduction of NOx from a burner and includes a burner body adjacent aport block. The present invention also includes a fuel passagewayconnecting a fuel source to a burner throat. Primary air jets areconnected to an air source and inject air into a primary combustionregion. This primary combustion region is in the burner throat. Theprimary air jets can be configured such that air is introduced into theprimary combustion region in a swirling manner. A dish surface islocated in the port block; the dish surface extending in an angledivergent with respect to a centerline extending through the burnerthroat. Finally, the present invention utilizes secondary air jetsconnected to an air source. These secondary air jets extend through theport block and inject secondary air into a secondary combustion regionlocated downstream from the primary combustion region.

The present invention also includes a method of reducing NOx emissionsfrom a burner, wherein fuel is taken from a fuel source and injectedinto a burner throat via a fuel passageway, and primary air is injectedfrom an air source into a primary combustion region in the burnerthroat. Further, this primary combustion is conducted in a fuel-richhighly vitiated environment which consumes available oxygen, limitingflame temperature and thermal NOx. Fuel is fed into the burner andproceeds to the throat where the primary air and fuel mix together toform the initial stage of combustion. A combustion reaction is initiatedin the burner throat. The preferable convergent, angled introduction ofthe air through the primary air jets creates a swirling cyclone patternthat hugs the walls of the port block and pulls and mixes the fuel andrecirculated products of combustion into the cyclone. After the primarycombustion step, the air/fuel mixture then enters a secondary combustionregion. Air is introduced into the secondary combustion region so as toallow the combustion process to complete. Products of combustion aredrawn into a vortex created by the swirling mixture of fuel and airduring the combustion process. The overall NOx production is therebyreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a single stage burner design according to theprior art;

FIG. 2 is a side view of a first embodiment according to the presentinvention;

FIG. 3 is a side view of a second embodiment according to the presentinvention;

FIG. 4 is a side view of a third embodiment according to the presentinvention;

FIG. 5 is a front view of the present invention illustrating a secondaryair jet hole configuration in a dish surface on a burner;

FIG. 6 is a front view of the present invention illustrating a furthersecondary air jet configuration in a hot face of the burner;

FIG. 7 is a front view of the present invention illustrating a stillfurther secondary air jet configuration in a hot face of the burner;

FIG. 8 is a side view of the present invention illustrating the use ofmultiple air supplies as applied to a non-regenerative burner;

FIG. 9 is a front view of the present invention illustrating a swirlingprimary air jet configuration;

FIG. 10 is a side view of the present invention illustrating a twodirection gas nozzle configuration;

FIG. 11 is a table illustrating the NOx emissions of the presentinvention versus conventional Coanda burners; and

FIG. 12 is a side view of a further embodiment according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 1, the design of a typical prior art burner includes aburner body 10, which houses an air passageway 14 and a fuel passageway12. The air passageway 14 may have an optional heat storing media 18area, depending upon the application. Fuel is introduced into the fuelpassageway 12, which directs the fuel through the burner body 10, andflows out through a fuel nozzle 22. All required combustion air entersthrough an air entrance 16, runs through the air passageway 14 andenters a combustion region through primary air jets 20. The burner body10 is fixed to a port block 42. The fuel and air initially mix in aburner throat 40 of the burner. Combustion occurs in the burner throat40 and continues into cup 26 and from these to a space surrounded by adish surface 28.

The present invention is an apparatus and method directed to anair-staged low-NOx burner. The first embodiment is illustrated in FIG.2. A liquid or gaseous fuel is introduced into the burner body 10through the fuel passageway 12 where it proceeds through the fuel nozzle22 into the burner throat 40 in a primary combustion stage 24. The airenters through the air entrance 16 where it may or may not pass throughthe heat storing media 18. The air flows through the air passageway 14and is split into primary air (i.e., the first air to be introduced tothe fuel), which exits through the primary air jets 20, and secondaryair, which exits through secondary air jets 34.

Due to the jet action and an angular orientation of the primary air jets20, the air enters the throat 40 in a swirling manner, illustrated asline 21 in FIG. 2. This swirling pattern is created by tangential forcesand causes the swirling air to travel along the dish surface 28 of theport block 42. This swirling and sticking phenomena (line 21) is calledthe “Coanda effect”, which also creates a negative vortex within thecenter of the air swirl. This negative vortex pulls the fuel stream andrecirculated products of completed combustion into the swirling air 21,mixing the components together. A preferable angular orientation of theprimary air jets 20 is illustrated in FIG. 9.

The combustion process is initiated by spark, pilot flame or anothersuitable method. Upon ignition, combustion occurs in the primarycombustion region 24. However, the fuel to primary air ratio is adjustedto ensure this combustion occurs under a highly vitiated fuel-richcondition. The fuel-rich condition allows the combustion process toconsume all available oxygen, disallowing complete combustion andpreventing creation of excess thermal NOx. Combustion under fuel-richconditions, coupled with the recirculated products of combustion pulledthrough the vortex, limits flame temperature and reduces the amount ofthermal NOx produced. Further, the “Coanda effect” causes the combustedmixture to continue along the surface of the burner throat 40, the cup26, and along the dish surface 28. This also provides a uniformtemperature and rotating flame within the port block 42. The dishsurface 28 extends in a divergent manner with respect to a centerline 35running through the longitudinal axis of the burner throat 40.Specifically, in the case of a planar or flat dish surface 28, thisangle of divergence a between the dish surface 28 and centerline 35 maybe between about 25° and 89° (i.e., ±5° on either end of the range) withthe preferred angle α between about 25° and about 50° (i.e., ±5°).

It is also envisioned that the dish surface 28 may have a continuouslyshifting angle of divergence α, resulting in a trumpet-like shape to thedish surface 28. As shown in FIG. 12, the angle of divergence α,measured between the centerline 35 and a line tangential to the rounded,bell-shaped dish surface 28, is continuously shifting. The trumpet-likeshaped dish surface 28 of FIG. 12 still allows for the required Coandaeffect, with enhancement of the Coanda effect by the secondary air jets34.

As the combusted mixture rides out of the cup 26 and into the dishsurface 28, the negative vortex continues to pull the products ofcombustion through the mixture from a furnace atmosphere into which theburner is firing. This mixture then encounters the secondary air jets34, which open into the dish surface 28. In a preferred embodiment,these secondary air jets 34 are oriented in a divergent manner. Asillustrated in FIGS. 2 and 3, the secondary air jets 34 are divergentwith respect to the centerline 35 running through the longitudinal axisof the burner throat 40. The angle of divergence β between the secondaryair jets 34 and centerline 35 may be between 1° and 89°, however theoptimal range is between about 15° and about 50° (i.e., ±5°). Largerangles could be beneficial to flame shape, but become difficult from aconstruction standpoint. It is envisioned that the burner throat 40, aswell as the fuel passageway 12 extend perpendicularly to the port block42 in a normal burner configuration. The divergent orientation of thesecondary air jets 34 encourages the same “Coanda effect”, furthermaintaining the negative vortex. Again, this negative vortex continuesto pull the air/fuel/products of combustion together into a homogenousmixture. This homogenous mixture, created by the use of the secondaryair jets 34, controls the combustion process and limits the flametemperature, thereby limiting the amount of thermal NOx produced in asecondary combustion region 38.

The primary air jets 20 and the secondary air jets 34 are controlled asto both velocity and air split ratio. Both of these characteristicscontrol the flame geometry, combustion pattern and the amount ofemissions emitted from the burner. Specifically, it is envisioned thatthe air split ratio be within the limits of 40/60 (primary air/secondaryair) to 75/25 (primary air/secondary air). As shown in FIG. 11, using a58% primary air/42% secondary air split ratio together with the abovedescribed invention, the burner NOx emissions are significantly reduced.However, this air split ratio can vary according to the use of ambientair and other variable factors.

Another embodiment of the present invention is illustrated in FIG. 3.This embodiment operates in substantially the same manner as the firstembodiment described above. However, as opposed to the secondary airjets 34 entering the dish surface 28 in a divergent orientation, thesecondary air jets 34 open at a hot face 30 in a divergent orientation.In this embodiment, the secondary combustion zone 38 is moved furtherinto the furnace. The swirling pattern and negative vortex are createddue to the angular entry of primary air. The flame geometry and overallcombustion process are altered in the new orientation. The mixing of thesecondary air with uncombusted partially-reacted fuel is further delayed(relative to FIG. 2), yielding further NOx reduction and increased flamediameter.

The third embodiment of the present invention is illustrated in FIG. 4.This embodiment operates in substantially the same manner as the firstembodiment described above. However, as opposed to the secondary airjets 34 entering the dish surface 28 in a divergent orientation, thesecondary air jets 34 enter the hot face 30 in an orientation parallelto the centerline 35 extending through the longitudinal axis of theburner throat 40. The flame geometry and overall combustion process arealtered in the new orientation. The flame will be more stable andproduce only slightly higher NOx (relative to the first and secondembodiments).

While the current air supply of primary and secondary air is describedas emanating from a common air source, it is also anticipated that asecond air source can be used to supply the secondary air jets 34. Forexample, the air may be supplied through direct connections topassageways in the port block 42. Using alternate air supplies allowfurther control of the flame geometry and combustion characteristicsthrough stoichiometric variation. As seen in FIG. 8, with application toa non-regenerative burner configuration, the secondary air jets 34 canbe supplied through a different air source. For example, a secondary airinlet 46 can be utilized, allowing secondary air to flow through asecondary air passageway 44 into the secondary air jets 34. This wouldallow the use of air with different qualitative and quantitativevariations than the primary air, yielding further control over theprocess. Still further, each of the secondary air jets 34 may haveidentical or different air sources from each other, allowing evengreater control of the process.

In another variation, the number and location of secondary air jets 34may be changed, affecting the flame geometry and combustion process.FIG. 5 shows a first secondary air jet configuration, using foursecondary air jets 34 equally spaced around the dish surface 28. FIG. 6shows a second secondary air jet configuration, using four secondary airjets 34 equally spaced around the hot face 30. FIG. 7 shows a thirdsecondary air jet configuration, using six secondary air jets 34 equallyspaced around the hot face 30. It will be apparent to those skilled inthe art that the number of secondary air jets 34 used, and theirrelative location, can vary. The preferred arrangement is with equallyspaced secondary air jets 34, however, non-uniformly spaced jets willfunction with minor change in NOx emissions.

Another arrangement for adjusting flame stability is seen in FIG. 10.Specifically, using a two-direction fuel nozzle 48 more evenlydistributes the fuel into the fuel/primary air mixture. This optionaladdition would create an even more homogenous mixture of fuel and air.

It will be evident to those of ordinary skill in the art that variouschanges and modifications may be made to the present invention withoutdeparting from the spirit and scope thereof. For example, the swirlingeffect in the burner throat 40 could be accomplished by swirling thefuel, instead of swirling the primary combustion air, as describedabove. It is therefore intended that the invention be limited only bythe attached claims, and equivalents thereof.

1. A method of reducing NOx emissions from a burner comprising the stepsof: (a) injecting fuel from a fuel source into a burner throat via afuel passageway; (b) injecting primary air from an ambient air sourcethrough a single connection into a primary combustion region located inthe burner throat, the ratio of fuel to primary air being such as tocreate a fuel rich mixture of fuel and primary air; (c) inducing aswirling effect upon the mixture of fuel and primary air in the burnerthroat; (d) combusting the mixture of fuel and primary air; (e) passingthe swirling mixture of fuel and primary air to a port block wherein,due to Coanda effect, at least part of the mixture of fuel and primaryair remains adjacent a dish surface in the port block; (f) injectingsecondary air from the same ambient air source as said primary air andthrough said single connection into a secondary combustion regionlocated downstream from the primary combustion region in an amount atleast sufficient to complete the combustion of the fuel; wherein thesecondary air is injected through at least one secondary air jetdiverging with respect to a centerline extending through the burnerthroat and wherein the air split ratio of primary air to secondary airis within the range 40/60 to 75/25; and further wherein an angle ofdivergence of the at least one secondary air jet is from about 15° toabout 50°, and is less than an angle of divergence of said dish surface;(g) drawing products of combustion into a vortex created by the swirlingmixture of fuel and air during the combustion process, thereby reducingNOx produced in the combustion process.
 2. The method of claim 1,wherein the swirling effect in the burner throat is induced byconfiguration of at least one primary air jet.
 3. The method of claim 2,wherein the secondary air is injected to the dish surface.
 4. The methodof claim 1, further comprising the step of passing the primary andsecondary air through a heat storing media.
 5. The method of claim 1,wherein the secondary air is injected to a burner hot face on the portblock.
 6. The method of claim 1, wherein the fuel is injected through afuel nozzle on an end of the fuel passageway.
 7. The method of claim 6,wherein the fuel nozzle is configured to inject the fuel in more thanone direction.
 8. The method of claim 6, wherein the fuel is caused toswirl in the burner throat via the fuel nozzle.